For many years the airports have been equipped with the Instrument Landing System known as the ILS. However, this landing system is about to be replaced by a new Microwave Landing System, known as the MLS which has recently received virtually worldwide acceptance. The first order for the ground portion of the system MLS has recently been placed by the FAA.
In view of the essential nature of aircraft landings, often considered to be the most critial of ordinary maneuvers, it is important to have a monitoring and backup system that provides an independent check during in-flight approaches to insure the absolute reliability of the data being provided by the principal landing system. This would be in addition to the usual ground based monitors which are used for checking course alignments, signal strengths, etc. of the signals radiated from the ground based landing system components. In the aircraft, other types of navigation techniques are constantly in use to check on the accuracy of the landing system, but none provides the high degree of reliability required during final approach.
One possible technique for providing an independent landing monitor system (ILM) for the critical landing operation is to provide a duplicate ground installation and to compare in the aircraft the guidance data provided by both ground facilities and if such data agrees within prescribed limits to proceed with the landing.
In ILS systems, it has been impractical to provide a second identical ILS as a back-up monitor at the same airport, principally because of the large size of the ILS antennas. Moreover, even though the antennas are much smaller in MLS systems because of the higher frequencies used, about 5000 MHz, it would be impractical to provide a second identical MLS at the same airport, because of the very high cost of the MLS systems, about $500,000 each.
A second ILM technique is to use airborne equipment already installed for other purposes to provide landing guidance data for such independent monitoring purposes.
The weather radar, either modified or unmodified and in cooperation with gound-based reflectors or beacons, has frequently been suggested for implementing an ILM, especially since weather radar is used when flying under bad weather (IFR) conditions.
This weather radar ILM concept is taught in my U.S. Pat. No. 3,243,816, which uses the airborne weather radar together with ground installed passive reflectors or alternatively with radar beacons of the frequency shift type, to display guidance paths for landing, or for landing monitoring purposes.
More recently, Assam in his U.S. Pat. No. 3,729,737 added a further teaching involving the detecting, by means of airborne radar, of plural tilted reflectors to generate glideslope guidance patterns for ILM purposes.
Gendreu et al in U.S. Pat. No. 4,103,300 suggests the use of weather radar and a ground beacon system for ILM purposes. However, his technique tends to require complex airborne instrumentation and therefore cannot use a standard weather radar, which is a major drawback.
My U.S. Pat. No. 4,429,312 overcomes the problems of requiring a non-standard weather radar by using a standard weather radar beacon for ILM purposes in a time sequence mode of operation.
The problem with all of the above noted weather radar uses for ILM purposes is that they require the use of a weather radar which may not be installed on all aircraft of interest. What is required then is an ILM that provides a completely independent monitoring function which is integrated within the framework of the landing guidance system that is being monitored and which does not require the installation of added airborne equipment. In particular this invention seeks to provide this type of ILM capability in the present worldwide standard MLS landing system.
An ILM for monitoring the progress of each landing at an MLS site is therefore desirable, whereby an aircraft can obtain truly independent confirmation of the MLS guidance data from ILM ground equipment which is integrated with the MLS ground equipment. In such a system the airborne derived ILM data will be truly independent, but can be based upon the use of the already installed MLS airborne equipment without requiring added airborne equipment.
MLS is a sequentially functioning system which can provide up to 15 different functions at different times in the MLS radiating sequence. These functions can be divided into two separate categories, one category providing guidance, and the other category providing to the aircraft data relating to that particular MLS installation, i.e. location of MLS equipment with respect to the runway, equipment status, type of services provided, etc. The MLS signal format currently includes both an auxiliary guidance function, and an auxiliary data function (not yet fully specified). These auxiliary unspecified functions are to accommodated future growth of the MLS system.
MLS is also a very flexible system wherein at some installations, as for example at a very busy airport, essentially all functions may be provided, whereas at smaller airports only some functions may be provided.
Each MLS function, when it is provided, i.e. radiated, is accompanied by an associated preamble identification code. Since each function has such an identification code, the functions can be radiated or provided at different or randomly selected times, not necessarily in any particular sequence. Specific sequences are however recommended in the ICAO SARPS (Standards and Recommended Practices) for installations that provide a particular combination of functions. In addition each particular function must be radiated at a certain minimum repetition rate consistent with the service that function performs, i.e. azimuth approach guidance must be provided at a rate consistent with aircraft/pilot response for a desired guidance performance.
The precision guidance functions of the MLS are provided by means of a narrow beam that scans the region in which precision guidance is being provided. The time between successive passages of the scanning guidance beams past the airborne receiving antenna is precisely measured by the airborne precision timing circuit and used to provide the desired angular guidance data.
The purpose of using scanned beams for localizer and elevation determinations in MLS, as distinguished from fixed beams as used in ILS, is to permit the approach and landing of aircraft along nonlinear courses having greater flexibility than straight-line paths, i.e. permitting curved azimuth and elevation approaches which are deemed especially useful at high traffic airports. Although a curved approach path may be useful at som e distance from touchdown, in order to be more certain of a safe landing, the aircraft will usually fly the last, most critial, portion of the approach to touchdown along the usual non-maneuvering straight-line centerline course. Moreover, during most approaches, the aircraft will still follow a relatively standard straight-line glidepath, typically a 3.degree. glideslope which is the same as used in ILS landings, prior to touchdown.
In addition to the use or radiation of the scanning beam to provide precision guidance, the MLS guidance function may also include the use of sequentially radiated fixed beams.
These fixed beams serve two separate purposes. One purpose termed OCI (out of course indication) is to suppress false courses outside the established MLS guidance region. These false courses might be caused by side lobes of the precision scanning beams. False course suppression is accomplished by radiating one or more fixed beams that provide greater signal strength, by a prescribed amount, than the side lobes of the scanning beam in the area in which it is desired to suppress possible false guidance courses. Up to six false course suppression beams can be radiated within the azimuth guidance function and up to two within the elevation guidance fuction.
The second purpose of using radiated fixed beams is to provide a clearance capability. Clearance beams are used in the MLS installations where the azimuth scanning beam does not scan the entire, normally prescribed, precision azimuth guidance region of plus of minus 40.degree. about the runway centerline, but scans only a portion of that region. In such installations these clearance beams are radiated left and right of the scanning beam precision coverage, but within the specified guidance coverage region. Measurement of the amplitudes of such beams will provide a fly/left, fly/right signal for use in the aircraft for intercepting the region in which precision proportional guidance is provided by the scanning beam. Both the OCI and the clearance beams are radiated at prescribed times within the time allocated to the guidance function within which they might be utilized.
It can be noted therefore that MLS is a sequentially operating syustem that can provide many different guidance functions in a very flexible building block configuration. In addition this flexibility is enhanced by providing auxiliary functions for unspecified future growth potential. Precision guidance data is provided by accurate measurements of the times when the scanning beam passes over the aircraft. In addition airborne amplitude measurements are also being made to determine the intensities of sequentially radiated fixed beams that may be utilized at some MLS installations to prevent false courses (OCI beams) and to aid in the acquisition of the precision guidance beams (clearance beams).